As most vehicles of this kind have two steerable wheels, and sometimes even more, reference will only be made in the following description to "the steerable wheels" so as not unduly to clutter up the description. But it will readily be seen that the present invention applies without change to vehicles having only one steerable wheel.
The driver of such a vehicle is obviously provided with a steering control member that he can actuate to cause the steerable wheels of his vehicle to pivot in relation to their pivotal axes, and hence to steer the vehicle. In most cases this member consists of a steering wheel which the driver can rotate about an axis. In other cases, this member consists not of a steering wheel but, for example, of a hand-actuated stick that the driver of the vehicle can move, parallel to itself or by causing it to pivot about an axis, in a plane which is, for example, perpendicular to the longitudinal axis of the vehicle.
In most vehicles of the above-defined kind, the slaving of the angular position of the steerable wheels to the position of the steering control member is performed by a purely mechanical device.
It has already been proposed to produce such slaving by means of a device having an electric or electronic slaving circuit that controls a motor mechanically linked to the steerable wheels in response to electric signals that are respectively representative of the pivotal angle of these wheels and of the position of the steering control member, these signals being supplied to the circuit by position sensors that are mechanically linked to said wheels and to said member, respectively.
In the remainder of this description, such a device will be termed "electric slaving device" and the electric or electronic circuit it comprises will be termed "slaving circuit".
In a steering control device using such an electric slaving device, there is no mechanical link between the vehicle's steerable wheels and its steering control member.
It should be noted that, throughout the present description, the term "mechanical link" between a first element and a second element, here the steerable wheels on the one hand and the steering control member on the other hand, must be understood to mean a functional link entirely made up of mechanical components and whereby any movement by one of the elements results in a corresponding movement by the other element, and not to mean the link that merely results from the fact that these two elements form part of the same vehicle.
A vehicle steering control device having an electric slaving device suffers from the major drawback of depriving the vehicle's driver of all control over the steering of the vehicle in case of faulty operation of the electric slaving device, the term faulty operation obviously applying also to a complete breakdown of the device. Such faulty operation of the electric slaving device could for instance be caused by a substantial drop, or by the complete fading out, of its supply voltage or by a failure of one of the components of the slaving circuit or of one of the position sensors.
An object of the present invention is to propose a vehicle steering control device which does not suffer from the drawback of the above-described known device, i.e. a device that enables the vehicle's driver to retain control of the steering of the vehicle even in the case of faulty operation of the electric slaving device.